This application claims the benefit of Japanese Application No. 11-254940, filed in Japan on Sep. 8, 1999, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical element, and more particularly, to a method for manufacturing an optical element which contains fluoride at least in its surface portions, and further relates to an optical element which shows little drop in transmissivity over elapsed time of use. Moreover, the present invention also relates to an optical apparatus equipped with an optical system that contains the above-mentioned optical element.
2. Discussion of the Related Art
Fluoride materials are transparent over a broad light wavelength range extending from the infrared region to the ultraviolet region. Especially at wavelengths shorter than 180 nm, most fluorides are transparent, while almost all oxide materials are opaque. As a result, fluorides are indispensable materials for optical elements used in the ultraviolet region.
Since fluoride optical elements possess the above-mentioned superior optical characteristic, such optical elements are widely used in optical apparatuses such as semiconductor exposure apparatuses, microscopes, binoculars, camera lenses, telescopes, eyeglasses and medical treatment devices, etc.
However, in conventional fluoride optical elements, the problem of a drop in transmissivity occurring over time during use, which does not occur in the case of quartz glass optical elements, has been encountered. This problem is especially severe in cases where the light that is used is more in the ultraviolet region than in the visible region. In order to alleviate this problem, it is necessary to clean the optical elements occasionally during use. Accordingly, in the case of optical apparatuses such as semiconductor exposure apparatuses or medical equipment apparatuses, that are equipped with optical systems incorporating such optical elements, the maintenance time required for this necessary occasional cleaning causes a drop in the operating efficiency of the apparatus. Thus, this problem is serious.
Accordingly, the present invention is directed to an optical element, a method for making the same, and an optical apparatus using the same that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
The object of the present invention is to solve the above-mentioned problems, and to provide a fluoride optical element which shows substantially little drop in transmissivity over time, a method for manufacturing this optical element, and an optical apparatus equipped with an optical system that incorporates one or more of this optical element.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the optical element invention includes a manufacturing method to provide an optical element containing a fluoride in at least its surface portions, and replacing F atoms at the surface by OH groups or O atoms, or both.
In another aspect, the optical element invention includes a manufacturing method which provides an optical element that contains fluoride in at least its surface portions, and imparts hydrophilic properties to the surface of an optical element.
In another aspect, the optical element invention includes a manufacturing method which provides an optical element that contains fluoride in at least its surface portions, and replaces F atoms at the surface portions by OH groups or O atoms, or both; wherein the surface portions have properties including at least one of substantial prevention of charge build-up and substantial prevention of charge retention.
In another aspect, an optical element containing fluoride has a surface having hydrophilic properties as a result of OH groups or O atoms, or both that have replaced F atoms at the surface.
In another aspect, the optical element invention includes a projection exposure apparatus which projects and exposes the image of a mask pattern on a substrate using optical systems that comprise an illumination optical system which illuminates the mask with ultraviolet light as exposing light, and a projection optical system which includes a fluoride optical element which comprises a surface having hydrophilic properties, and which forms the above-mentioned mask pattern image on the substrate.
In another aspect, the optical element invention includes a projection exposure apparatus which projects and exposes the image of a mask pattern on a substrate using optical systems that comprise an illumination optical system which includes a fluoride optical element which comprises a surface having hydrophilic properties as a result of OH groups or O atoms, or both that have replaced F atoms at the surface, and which illuminates the mask with ultraviolet light as exposing light; and a projection optical system which forms the above-mentioned mask pattern image on the substrate.
In a further aspect, the optical element invention includes a projection exposure apparatus which projects and exposes the image of a mask pattern on a substrate using optical systems that comprises an illumination optical system that includes a fluoride optical element having hydrophilic properties, and which illuminates the mask with ultraviolet light as exposing light; and a projection optical system that includes a fluoride optical element having hydrophilic properties, and which forms the above-mentioned mask pattern image on the substrate.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.